The present invention relates to optical wavelength couplers and, more particularly to broadband wavelength-division multiplexers and demultiplexers.
A wavelength-division multiplexing (WDM) technique is a technique to combine (multiplex) input signals at different wavelengths so that they can be coupled into the same fiber or waveguide. The resulting signal is called a WDM signal. The optical device that multiplexes signals at different wavelengths into a WDM signal is called a wavelength-division multiplexer, whereas the one that demultiplexes a WDM signal into signals at different wavelengths is called a wavelength-division demultiplexer. The term broadband thereby refers to multiplexed signals within a broad spectral range such as from 1.25 xcexcm to 1.35 xcexcm in the 1.3 xcexcm wavelength band.
Low cost broadband wavelength-division multiplexers and demultiplexers with inputs from more than one wavelength band may find applications in bi-directional transmitter-receiver devices in a WDM communication system, where different wavelength bands are used for the two propagation directions of the signals. They also may be used to add an additional wavelength to supplement capacity at another wavelength and may be used for many other applications.
Generally, one way to lower the cost is to reduce the size of a broadband wavelength-division multiplexer or demultiplexer. Another way is to design a broadband wavelength-division multiplexer or demultiplexer that can be monolithically integrated with other optical devices such as lasers and photodetectors. The reason that monolithic integration can lower the cost is that the manufacturing and maintaining cost is generally cheaper.
In an article xe2x80x9cLow-Loss 1xc3x972 Multimode Interference Wavelength Demultiplexer in Silicon-Germanium Alloyxe2x80x9d, IEEE Photonics Technology Letters, Vol. 11. No. 5, May 1999, pp. 575-577, by Li et al. proposed the application of the multi-mode interference (MMI) principle to a wavelength-division demultiplexer that can be monolithically integrated with silicon-based devices. The MMI principle is based on the self-imaging principle, wherein an input signal to a MMI waveguide is reproduced in single or multiple images at periodic intervals along the propagation direction of the MMI waveguide. Unfortunately, the length of the MMI waveguide in this demultiplexer is approximately 2,600 xcexcm.
In accordance with the invention, the length of a MMI waveguide used in a broadband wavelength-division multiplexer having inputs from different wavelength bands can be shorter than 2600 xcexcm.
The multiplexer can be implemented in InGaAsP/InP, silica on silicon and many other materials to achieve the same multiplexing/demultiplexing with a device length shorter than 2600 xcexcm.
The invention is about a wavelength multiplexer/demultiplexer with compact dimension and complete multiplexing/demulitplexing for one wavelength band, but only partial multiplexing/demultiplexing for the other wavelength band. The partial multiplexing/demultiplexing is an important step, since it allows to reduce dimension of the MMI wavelength multiplexer/demultiplexer considerably in comparison with a devise that provides complete multiplexing/demultiplexing.
One embodiment of the invention is a broadband wavelength-division multiplexer that comprises two input waveguides for two input signals, each of which is from a different wavelength band, a MMI waveguide for producing an output WDM signal from the two input signals, and a WDM output waveguide for coupling out the output WDM signal. Optionally, at least another output waveguide can be used to trap all or some power not coupled to produce the output WDM signal.
Another embodiment of the invention is a broadband wavelength-division demultiplexer for demultiplexing an input WDM signal of wavelengths from two wavelength bands. The broadband wavelength-division demultiplexer comprises one input waveguide, a MMI waveguide for producing two output signals where the MMI waveguide couples partial power from one of the two wavelength bands and substantially no power from the other wavelength band to produce one of the two output signals at one of the wavelength bands, and two output waveguides for providing the two output signals.
It is evident, that the multiplexer and or demultiplexer operation can be implemented into the same device.